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Although I'm no expert in bonsai, I have had two of the more hardy trees indoors, a Chinese Elm (Ulmus parvifolia) and a Weeping Fig (Ficus benjamina) for over ten years now, and both are very healthy.

I replace the soil every couple of years and, during the growing season, fertilize them every fortnight with a standard feed for houseplants (Nitrogen 10.6%, Phosphorus 1.9%, Potassium 1.4%) at half strength (after replacing the soil, I normally wait about six weeks before fertilizing, since there are enough nutrients for their needs in the fresh soil).

Many people who are new to bonsai mistakenly believe that it's necessary to withhold fertilizer to prevent their trees from growing too big, whereas, in fact, growth restriction is achieved through root and foliage pruning, not by denying them the fertilizer they need to remain healthy.